The present invention generally relates to a signal reproduction apparatus, and more particularly to an apparatus reproducing a signal from a rotary recording and reproducing medium on which a modulated signal is recorded.
When information is recorded on a rotary recording and reproducing medium such as an optical magnetic disc and an optical disc (hereinafter termed a disc), an original data is modulated in accordance with a predetermined role, and data which is obtained by modulating the original data is recorded on the disc.
For example, a modulation method such as a four out of eleven modulation (4/11 modulation) and a four out of fifteen modulation (4/15 modulation) is proposed.
In data modulated by using the four out of eleven modulation method, one data block of the data has eleven bits, and each of the eleven bits corresponds to a cycle of a predetermined clock pulse. Then, the data in which four bits out of eleven bits are active (for example, "1") in the one data block, as shown in FIG. 1(A), is recorded on the disc. In the four out of fifteen modulation, as in the case of the four out of eleven modulation, one data block has fifteen bits, and the one data block is expressed so that four bits out of the fifteen bits are active (for example, "1").
In such modulations, in general, one data block has n bits and is expressed so that m bits out of n bits are active, where m and n(&gt;m) are integers.
A signal reproduction apparatus reproduces data by one data block from the disc on which data modulated as shown in FIG. 1(A) is recorded.
FIG. 2 shows an example of a conventional signal reproduction apparatus reproducing data which is modulated in accordance with the four out of eleven modulation.
A detection signal which is detected from the disc by an optical head (which is not shown in FIG. 2) inputs to an input terminal 10. The detection signal, for example, has a wave form as shown in FIG. 1(B). The detection signal is supplied through the input terminal 10 to a non-inverting terminal(+) of a comparator 11. The comparator 11 compares the detection signal with a reference voltage which is applied to an inverting terminal(-) thereof. An output signal of the comparator 11 has a high level (V volt) when a level of the detection signal is greater than or equal to the reference voltage, and the output signal thereof has a low level (0 volt) when the level of the detection signal is less than the reference voltage. The output signal of the comparator 11 inputs to a low pass filter 13. The low pass filter 13 integrates the output signal of the comparator 11 and outputs an average voltage obtained by integrating the output signal of the comparator 11. The average voltage is applied to a differential amplifier 14. The differential amplifier 14 outputs a voltage corresponding to a difference between the average voltage from the low pass filter 13 and a reference voltage V.sub.REF applied through a terminal 15 from a controller (which is not shown in FIG. 2). The reference voltage V.sub.REF is, for example, equal to 4.times.V/11 volts in the case of the four out of eleven modulation (4/11 modulation). V expresses a high level voltage of the output signal of the comparator 11. Then, an output voltage from the differential amplifier 14 is applied to the inverting terminal(-) of the comparator 11 so that the average voltage output from the low pass filter 13 becomes 4.times.V/11. Thus, the comparator 11 inputting the detection signal such as shown in FIG. 1(B) outputs a pulse signal having a wave form in which an upper level is V volts and a lower level is 0 volt, as shown in FIG. 1(A). That is, the signal reproduction apparatus shapes the detection signal such as that shown in FIG. 1(B) and outputs a reproduction signal which has a rectangular wave form such as that shown in FIG. 1(A). The signal output from the comparator 11 is applied through an output terminal 12 of this signal reproduction apparatus to a demodulation circuit (which is not shown in FIG. 2).
A method of reproducing the signal by using a circuit such as that shown in FIG. 2 is termed a comparator reproduction method.
FIG. 3 shows another example of a conventional signal reproduction apparatus reproducing a signal which is also modulated in accordance with the four out of eleven modulation. A method of reproducing the signal by a circuit such as that shown in FIG. 3 is termed a maximum likelihood reproduction method.
A detection signal which is detected from the disc by an optical head and has a wave form such as that shown in FIG. 1(B) inputs to an input terminal 20. The detection signal is converted to digital data by an analog-to-digital converter 21 in synchronism with a predetermined clock signal which has a cycle corresponding to one bit in one data block of the modulated data. That is, the analog-to-digital converter 21 samples the detection signal in synchronism with the clock signal. Eleven sampling values which are sampled for one data block of the data are stored in a memory 22. Each of the cycles of the eleven samplings corresponds to one bit in the one data block. Four sampling values of eleven sampling values are stored in registers 24a through 24d, then, each of the remaining seven sampling values in the memory 22 is compared with each of the four sampling values stored in the registers 24a through 24d by each of four comparator 23a through 23d. When each of the remaining seven sampling values in the memory 22 is greater than one of the four sampling values stored in the registers 24a through 24d, each of the sampling values in the memory 22 is newly stored in a corresponding one of the four registers 24a through 24d. Then, when comparison processes in the comparators 23a through 23d over all of the seven sampling values finish, the four registers 24a through 24d have the greatest four sampling values among the eleven sampling values. A logic circuit 25 controls the four comparators 23a through 23d and four registers 24a through 24d so that the processes described above are performed, and stores information related to a position of the bit in the one data block of the recording data corresponding to each of the greatest four sampling values in the registers 24a through 24d. When the process for one data block of the data has finished, all of the four registers 24a through 24d are reset and "0" data is stored in each of the four registers 24a through 24d. Then, the logic circuit 25 outputs a reproduction signal, for every data block, in which only four blocks corresponding to the greatest four sampling values stored in four registers 24a through 24d are active, as shown in FIG. 1(A). The reproduction signal from the logic circuit 25 is supplied through an output terminal 26 to the demodulation circuit.
In the conventional signal reproduction apparatus to which the comparator reproduction method is applied, a circuit structure is simple. However, this conventional signal reproduction apparatus is not accurate for reproducing the signal, so that if a condition for reproducing a signal from the disc becomes worse errors occur in the reproduction signal. That is, in the case of the four out of eleven modulation, there is a disadvantage in that in the reproduced signal corresponding to the one data block, the number of active bits are more or less than four.
The other conventional signal reproduction apparatus to which the maximum likelihood reproduction method is applied is very accurate so that no errors of reproduction occur. However, in the other conventional signal reproduction apparatus, it is necessary to use many parts such as comparators and registers, and a logic of the logic circuit 25 is also complex.